1. Field of the Invention
This invention relates to Christiansen cells and methods of stabilizing the color images obtained with such cells and more particularly to such cells and methods which use photopolymerizable monomers to imagewise modulate the Christiansen effect.
2. Description of the Prior Art
A. Christiansen Cell Prior Art
It is well known that, in general, a train of light waves changes direction, i.e., is refracted, when it crosses a boundary separating two media of different indices of refraction and that, except in special cases, only a portion of the incident light passes into the second medium, the remainder being reflected. The directions of propagation of both the reflected and transmitted waves are different from that of the incident wave.
These phenomena are exploited in the Christiansen filter which produces a narrow band pass of color and which has application in the infrared and ultraviolet as well as in the visible region. This filter was first described by C. Christiansen in 1884. The Christiansen filter consists of a finely divided transparent material suspended in an optically homogeneous medium; the constituents are chosen so that they have different but intersecting dispersion curves, i.e., the refractive indices are identical at a particular wave length .lambda..sub.c in or near the visible region, but differ for all other wave lengths. The filter is optically homogeneous for light of wave length .lambda..sub.c, i.e., such light is unaffected and passes through the filter without deviation or reflection. However, for all other wavelengths the filter is optically heterogeneous and such light is scattered as a result of the refraction and reflection which occur at the particle-medium interfaces. The degree of scattering for a given wavelength depends on the difference in the two refractive indices at that wavelength so that greater scattering is expected for wavelengths farther from .lambda..sub.c. Consequently, the transmission curve for the unscattered light exhibits a maximum at .lambda..sub.c.
The only disclosure of the formation of color images by imagewise modulation of the Christiansen effect is contained in the commonly assigned patent application U.S. Ser. No. 515,406, filed Oct. 16, 1974, to Waring now U.S. Pat. No. 3,951,520. A Christiansen cell is formed with a mixture of two immiscible condensed phases having dispersions of refractive index different from each other but with indices of refraction matching at one wavelength in the vicinity of the visible region of the spectrum. The refractive index of one phase (or both) is then locally changed in accordance with a predetermined image so that the matching wavelength changes. Light is passed through the cell and the scattered and unscattered components of the light emerging from the cell are separated and either component is then focused to form a colored image. The use of photopolymerizable monomers is disclosed as one method of achieving imagewise modulation of the Christiansen effect with the various degrees of polymerization causing corresponding shifts in the dispersion curve of the continuous phase. The sharpness of images formed with such cells is gradually lost due to the combined diffusion of polymer formed and monomer across the image boundaries.
B. Photopolymerization Prior Art
This prior art involves the photopolymerization of di- or multifunctional monomers to form scatter image systems. Imagewise exposure of these materials to actinic light results in regions of insoluble polymer that separate from the medium and subsequent image formation is dependent on the scattering of incident light by these polymerized regions. The scattering efficiency depends on the physical differences, in particular, the difference in the indices of refraction of the particulate, insoluble, photopolymerized material and the unexposed material. For example, Rust, U.S. Pat. No. 3,649,495 discloses a method of polymerizing vinyl monomers using a photo-redox system to form generally opaque images. Rust, U.S. Pat. No. 3,615,627 describes methods for preparing clear solutions of multivalent metal salts of acrylic acids with acrylic and methacrylic acids and for preparing clear photosensitive films from these mixtures. The amount of material polymerized, and therefore the optical density of a given region, depends upon the exposure that region receives. The use of a multivalent metal salt which increases the difference in the index of refraction of the polymer and that of the unexposed solution is emphasized. In another example of this general approach, Rust, U.S. Pat. No. 3,726,688 describes compositions which use acrylic materials that can be polymerized both by photoinitiation and by acidcatalyzed condensation. After photopolymerization has been used to form a photoimage of light scattering polymer, the photosensitive composition is heated to cause a latent catalyst to form an acid to condensation polymerize the remaining monomer. All of the monomer is polymerized, i.e., both the image and background areas, and this invention relies on the physical difference between the fixed photopolymerized and the condensation-polymerized monomers to produce an image. The invention thus provides a method for stabilizing such images with respect to further exposure to actinic light by densensitizing the areas which were unexposed when the image was formed. All of the above examples provide black and white images although Rust, U.S. Pat. No. 3,726,688 discloses the use of certain photochromic dyes to provide colored systems.
C. Photopolymerization in Printing Plates
Printing plate prior art involves the use of cross-linking to stabilize photopolymer images used as printing plates, in reproduction processes, etc. The photosensitive material consists of di- or multifunctional monomers, multifunctional monomers in conjunction with monofunctional monomers, or in some cases these monomers in conjunction with polymers. In order to maximize the difference in whatever property is used to distinguish the light-exposed and the unexposed regions, it is desirable to have complete polymerization, i.e., complete incorporation of the monomers into chains, in the light-exposed regions. For example, one method of making relief images for use as printing plates is to form a rigid, insoluble polymerized mass, i.e., a resist that cannot be washed away with the soluble unexposed material. Typical examples of such photopolymerizable compositions and elements useful in preparing printing reliefs are given below.
Plambeck, U.S. Pat. No. 2,760,863 discloses the use of a photopolymerizable material composed of a single monomer, admixtures of monomers, or monomers containing addition polymers. Preferably, there is an appreciable proportion of ethylenically-unsaturated polymerizable material containing a plurality of polymerizable linkages per molecule, i.e., a cross-linking agent. As a result, material in the exposed areas is substantially completely polymerized and rendered insoluble in a solvent which is suitable for washing away the unexposed material. Martin and Barney, U.S. Pat. No. 2,927,022 disclose the use of photopolymerizable material composed of an addition-polymerizable, ethylenically unsaturated component and a polymer component, preferably a cellulose ether or ester containing lateral acidic groups, which is water or aqueous base-soluble thereby allowing these convenient materials to be used as solvents for washing away the unexposed regions. Hard, sharp, relief images result. Skoultchi, U.S. Pat. No. 3,574,617 discusses the use of photosensitive compositions that are prepared from a copolymer containing moieties derived from at least one vinyl monomer and at least one ethylenically unsaturated derivative of a substituted benzophenone. Cross-linking and insolubilization occur on exposure of the polymer film.
Other reproduction processes rely on the difference of the stick or transfer temperatures of the unexposed, unpolymerized material and the exposed, polymerized material. Upon heating to a temperature above the stick temperature of the unexposed material, but below that of the exposed material, a relief image corresponding to the unexposed material can be transferred to a receptive support. Complete polymerization in the exposed areas is desirable to insure a rigid, adhesive material. Burg and Cohen, U.S. Pat. No. 3,060,023 disclose this process and teach that the additionpolymerizable component is preferably an ethylenically unsaturated monomer having two terminal ethylenic groups.
Colgrove, U.S. Pat. No. 3,353,955 describes still another property which can be used to separate the exposed polymerized and the unexposed unpolymerized compositions. The photopolymerizable composition is comprised of a suitable binder and an ethylenically unsaturated monomer and is laminated between two supporting films at least one of which is transparent. The photopolymerizable composition is further characterized in that at room temperature in its polymerized state it has greater adhesion for one support than the other and in its unpolymerized state it has greater adhesion for the second support than for the first. After exposure the supporting films are separated by peeling them apart and the exposed areas adhere to the one support to form a negative image while the unexposed areas adhere to the other support to form a positive image. Schwerin, U.S. Pat. No. 3,234,021 describes a photosensitive coating consisting of a polymerizable vinyl monomer such as acrylamide and a cross-linking agent such as N,N'-methylene bisacrylamide. After exposure, the coating is moistened and squeezed against another support such as paper to effect transfer to the second support of the unpolymerized portions. The cross-linking agent increases the hardness of the exposed regions so that they will not be transferred to the second support.
In all of the photopolymerization work described above the goal is to maximize the differences between the exposed and the unexposed regions and this is accomplished by effecting substantially complete polymerization in the exposed regions. There is no objective for producing partially polymerized regions nor for stabilizing such an image.
In prior art relating to a different aspect of the instant disclosure, Pazos (in a commonly assigned patent application Ser. No. 550,660, filed Feb. 18, 1975,) discloses the addition of a nitroso dimer to a photopolymerizable composition to inhibit thermal polymerization. The nitroso dimer, which is a noninhibitor of free-radical polymerization, thermally dissociates to nitroso monomer which is an inhibitor. There is no mention of the use of this dimer to stabilize images against change due to postexposure polymerization. For most of the photopolymerization applications in which the exposed areas are completely polymerized and the unexposed unpolymerized areas are subsequently removed, post-exposure polymerization is no problem.